The present invention relates to a method of measuring the negative ion density of a plasma, a plasma processing method and a plasma processing system.
An electron cyclotron resonance plasma processing method (ECR plasma processing method) that produces a microwave discharge by utilizing absorption of the energy of a microwave by electrons in cyclotron motion by resonance has become noticed as a method of using a plasma for a film forming process or an etching process in recent years. The ECR plasma processing method is capable of producing a high-density plasma by electrodeless discharge in a high vacuum, of carrying out a rapid surface treatment process and of preventing the contamination of wafers.
A conventional ECR plasma processing system for carrying out an ECR plasma process will be described by way of example with reference to FIG. 7 as applied to a film forming process. Referring to FIG. 7, a microwave of, for example, 2.45 GHz is supplied through a waveguide, not shown, into a plasma producing chamber 9A and, at the same time, a magnetic field of, for example, 875 G is applied to the plasma producing chamber 9A by a solenoid 90 to convert a plasma producing gas, such as Ar gas, into a high-density plasma by the interaction (resonance) of the microwave and the magnetic field. A reactive gas, such as C4F8 gas, is activated by the plasma to produce active species. The active species is used for the simultaneous execution of a sputter etching process for etching a silicon wafer W mounted on a susceptor 91 connected to a high-frequency power supply 92 for applying a high-frequency bias voltage to the susceptor 91, and a film deposition process. The sputter etching process and the film deposition process, which are contrary to each other, are controlled so that the film deposition process is dominant for eventual film deposition.
The inventors of the present invention believe that the measurement of the negative ion density of the plasma is important for plasma processing. The ground of the belief will be described hereinafter.
For example, when a fluorine-containing carbon film (CF film) is used as a layer insulating film, a plasma produced by ionizing a CF gas is used for film formation. Ar gas, for instance, is added to the CF gas to stabilize the plasma. It is know from the respective measured electron temperatures and electron densities of a first plasma produced by ionizing only Ar gas and a second plasma produced by ionizing a mixture of Ar gas and a CF gas, such as C4F8 gas that the first and the second plasma are the same in electron temperature and that the electron density of the second plasma produced by ionizing the mixture is smaller than that of the first plasma produced by ionizing only Ar gas. A plasma is neutral and therefore,
ni+=ne+nixe2x80x83xe2x80x83(1)
where ni+ is positive ion density, ne is electron density and nixe2x88x92 is negative ion density.
It is known from the comparison of Expression (1) with the foregoing phenomenon that the fact that electron temperature does not change signifies that ni+ does not change, and the fact that ne decreases signifies that nixe2x88x92 decreases; that is, negative ions are produced when C4F8 is added to Ar gas.
In the foregoing ECR plasma processing system, there are many unknown actions of the plasma. Since the microwave and the magnetic field are involved, it is difficult to produce a uniform plasma over the surface of the wafer. It is known through the examination of the results of processing, such as the intrasurface film thickness distribution, of wafers processed under the same process conditions, that, in some cases, the wafers are different from each other in intrasurface film thickness distribution. The inventors of the present invention notice negative ions as one of factors that make the control of the condition of the plasma difficult. If the plasma has an excessively large negative ion density, the effective radicals of the plasma decreases. It is considered that an excessively large negative ion density affects bias adversely. Since negative ion density is dependent on the condition of the inner surface of walls of a processing vessel defining a processing chamber, the inventors of the present invention consider that a control loop must include negative ion density as a parameter.
Generally, the negative ion density nixe2x88x92 is determined by measuring positive ion density ni+ and electron density ne by a measuring method using a Langmuir probe, and calculating the negative ion density nixe2x88x92 by using Expression (1). This measuring method will be briefly described. A probe is inserted in a plasma, voltage VP is applied across the probe, and an anode or a cathode serving as a discharge electrode for producing a plasma, and ni+ and nixe2x88x92 are determined on the basis of current IP that flows through the probe when the voltage VP is changed.
FIG. 8 is a graph showing the dependence of the current IP on the voltage VP. The voltage VP is applied across the probe and the electrode connected to the probe. As the voltage VP increases toward the positive side, the current IP is saturated. A saturation current Ies in this state is expressed by Expression (2).
Ies=(e/4)xc2x7nexc2x7(8kxc2x7Te/xcfx80me)xc2xdxc2x7Axe2x80x83xe2x80x83(2)
As the voltage VP is decreased toward the negative side, the current IP is saturated. Saturation current Iis in this state is expressed by Expression (3).
Iis={e/exp[xc2xd]}xc2x7ni+xc2x7(kxc2x7Te/xcfx80mi)xc2xdxc2x7Axe2x80x83xe2x80x83(3)
In Expressions (2) and (3), e is elementary electric charge, k is Boltzmann constant, Te is electron temperature, me is the mass of and electron, mi is the mass of an ion, A is the effective collecting area of the probe for collecting ions and electrons. Generally, the area A is equal to the surface area S of a metal part of the probe.
The electron density ne is known from Expression (2), the positive ion density ni+ is known from Expression (3) and hence the negative ion density nixe2x88x92 is known from Expression (1). Electron temperature Te can be determined on the basis of the gradient of a section of the IP-VP curve in a region where the voltage VP is positive.
Generally, the negative ion density of the plasma can be thus measured. However, this method is not applicable to a plasma produced by ECR. Since an ECR plasma processing system does not have discharge electrodes, a base end part of a probe 100 is connected through a variable-voltage power supply 200 to a ground kept at a ground potential as shown in FIG. 9. Since a magnetic field B is created around the probe 100, the effective collecting area A in Expression (2) is a surface area Sxe2x80x2 smaller than the surface area S of the metal part of the probe, because the Larmor radius of electrons in a magnetic field is small, electrons wind round lines of magnetic force, and a portion of the surface of the metal part is shaded from a flux of electrons, so that the collecting area is reduced. Consequently, the positive saturation current Ies when the magnetic field B is created around the probe is lower than that when any magnetic field is not created around the probe as shown in FIG. 8. However, the saturation current Ies cannot be determined because Sxe2x80x2 is unknown. Since the collection area is S in Expression (3), Iis cam be determined. Thus, although the positive ion density ni (ni+) can be measured by a measuring method using the probe when the magnetic field is created, the electron density ne cannot be determined by the measuring method using the probe.
In the present state of art, the negative ion density nixe2x88x92 is determined by measuring the electron density ne by a microwave interferometer using change in the refraction of a microwave, and using the measured electron density ne and the positive ion density ni+ measured by a method using the probe.
However, the method of measuring the electron density ne by using the microwave interferometer requires troublesome operations and hence the method is not suitable for real-time measurement and needs an expensive instrument. Another method irradiates a plasma with light to make negative ions eject electrons, measures the amount of the electrons and determines the amount of negative ions on the basis of measured amount of the electrons. However, the accuracy of this method is not satisfactory because it is not sure whether all the negative ions eject electrons.
The present invention has been made in view of the foregoing problems and it is therefore an object of the present invention to provide a method capable of simply measuring the negative ion density of a plasma, particularly, the negative ion density of a plasma produced by electron cyclotron resonance.
Another object of the present invention is to provide a technique capable of minutely controlling processing state in processing a workpiece by a plasma produced by electron cyclotron resonance.
The principle of the present invention will be described.
It is a problem in the conventional measuring method using the Langmuir probe when a magnetic field is created around the Langmuir probe that the effective collecting area A is not equal to the surface area of the metal part of the probe and is equal to an area Sxe2x80x2 smaller than the surface area S and hence Expression (2) cannot be used.
The present invention originates in noticing that the effective collecting area A cannot be used as the surface area S in a magnetic field and may be equal to the unknown area Sxe2x80x2, the area Sxe2x80x2 is a constant unaffected by the power of the microwave.
The inventors of the present invention produced a plasma by ionizing, for example, Ar (argon) gas, calculated saturation currents Ies and Iis by using Expressions (2) and (3), examined the mode of change of the ratio Iis/Ies when the power of the microwave is changed, and found that the ratio Iis/Ies is fixed regardless of the change of the power of the microwave. That is, the ratio Iis/Ies which is a function of S/Sxe2x80x2 is a constant and hence Sxe2x80x2 is a constant. Accordingly, Sxe2x80x2 can be eliminated by calculating the ratio between a saturation current Ies when a plasma is produced by ionizing Ar gas and a saturation current Ies when a plasma is produced by ionizing C4F8 gas. The present invention has been made on the basis of this knowledge.
Suppose that a positive voltage relative to the ground is applied to the probe, a first measured saturation current Ies measured in a first plasma produced by ionizing a first gas is Ies1 and a second measured saturation current Ies measured in a second plasma produced by ionizing a second gas is Ies2. Similarly, suppose that ion density ni, electron density ne, ion mass mi and electron temperature Te with a subscript xe2x80x9c1xe2x80x9d denotes those obtained by measuring the first plasma of the first gas, and ion density ni, electron density ne, ion mass mi and electron temperature Te with a subscript xe2x80x9c2xe2x80x9d are those obtained by measuring the second plasma of the second gas. The first gas, i.e., an inert gas, is, for example Ar gas. The second gas for producing negative ions is C4F8 gas.
Expression (3) mentioned in the description of the background of the invention is rewritten according to this rule to provide Expressions (4) and (5) respectively expressing Iis1 and Iis2.
Iis1={e/exp[xc2xd]}xc2x7ni1+(kxc2x7Te1/xcfx80mi1)xc2xdxc2x7Sxe2x80x83xe2x80x83(4)
Iis2={e/exp[xc2xd]}xc2x7ni2+(kxc2x7Te2/xcfx80mi2)xc2xdxc2x7Sxe2x80x83xe2x80x83(5)
Therefore, the ratio Iis2/Iis1 is expressed by:
Iis2/Iis1=(ni2+/ni1+)xc2x7(Te2/Te1)xc2xdxc2x7(mi1/mi2)xc2xdxe2x80x83xe2x80x83(6)
Therefore, the ratio ni2+/ni1+ is expressed by:
ni2+/ni1+=(Iis2/Iis1)xc2x7(Te1/Te2)xc2xdxc2x7(mi2/mi1)xc2xdxe2x80x83xe2x80x83(7)
Expression (2) mentioned in the description of the background of the invention is rewritten according to the rule to obtain Expressions (8) and (9).
Ies1=(e/4)xc2x7ne1xc2x7(8kxc2x7Te1/xcfx80me)xc2xdxc2x7Sxe2x80x2xe2x80x83xe2x80x83(8)
Ies2=(e/4)xc2x7ne2xc2x7(8kxc2x7Te2/xcfx80me)xc2xdxc2x7Sxe2x80x2xe2x80x83xe2x80x83(9)
Therefore, the ratio ni2+/ni1+ is expressed by:
Ies2/Ies1=(ne2/ne1)xc2x7(Te2/Te1)xc2xdxe2x80x83xe2x80x83(10)
Therefore, the ratio ne2/ne1 is expressed by:
ne2/ne1=(Ies2/Ies1)xc2x7(Te1/Te2)xc2xdxe2x80x83xe2x80x83(11)
Expression (12) is formed from Expression (1) mentioned in the description of background of the invention, and Expression (13) is obtained by dividing both sides of Expression (12) by ni1+.
ni2+=ne2+nixe2x88x92xe2x80x83xe2x80x83(12)
                                                                        (                                                      n                    i2                    +                                    /                                      n                    i1                    +                                                  )                            =                                                (                                                            n                      e2                                        /                                          n                      e1                                                        )                                +                                  (                                                            n                      i                      -                                        /                                          n                      i1                      +                                                        )                                                                                                        =                                                (                                                            n                      e2                                        /                                          n                      e1                                                        )                                +                                  (                                                            n                      i                      -                                        /                                          n                      e1                                                        )                                                                                        (        13        )            
The foregoing expressions is rewritten on an assumption that ni1+=ne1 because the plasma produced by ionizing Ar gas does not contain any negative ion.
Expression (14) is obtained by substituting ni2+/ni1+ expressed by Expression (7) and ne2/ne1 expressed by Expression (11) into Expression (13). Magnetic fields of the same intensity and microwaves of the same frequency and the same power are used for ionizing the first and the second gas. The electron temperatures Te1 and Te2 are scarcely different from each other and Te1/Te2 is approximately 1. Therefore,
(Iis2/Iis1)xc2x7(mi2/mi1)xc2xd≈(Ies2/Ies1)+(nixe2x88x92/ne1)xe2x80x83xe2x80x83(14)
A method of measuring the negative ion density of a plasma according to the present invention determines negative ion density nixe2x88x92 by using Expression (14) and measured values of Iis1, Iis2, Ies1 and Ies2.
Although the negative ion density of the plasma produced by ionizing only C4F8 gas has been discussed above, practically, a mixed gas prepared by adding Ar gas to C4F8 gas is ionized to produce a stable plasma when forming a film or a semiconductor wafer or when etching a film formed on a semiconductor wafer. In such a case, mi2 of Expression (14), i.e., the reduced mass of dominant positive ions among the positive ions of the mixed gas prepared by adding Ar gas to C4F8 gas, is equal to mi1xc2x7xcex1+m(CF2+)xc2x7(1xe2x88x92xcex1), where M(CF2+) is the mass of CF2+ and xcex1 is the ratio of the flow rate of Ar gas to that of the mixed gas.
When a C4F8 plasma contains a plurality of kinds of main ions, i.e., when the C4F8 plasma contains CF4+ ions, C2F4+ ions, . . . as main ions, ni2+ is expressed by:
ni2+=ni(Ar+)+ni(CF4+)+ni(C2F4+)xe2x80x83xe2x80x83(15)
Therefore, the reduced mass mi2 in the mixed gas is expressed by:                               m          i2                =                                                                              n                  i                                ⁡                                  (                                      Ar                    +                                    )                                            ·                              m                i1                                      +                                                            n                  i                                ⁡                                  (                                      CF                    4                    +                                    )                                            ·                              m                ⁡                                  (                                      CF                    4                    +                                    )                                                      +                                                            n                  i                                ⁡                                  (                                                            C                      2                                        ⁢                                          F                      4                      +                                                        )                                            ·                              m                ⁡                                  (                                                            C                      2                                        ⁢                                          F                      4                      +                                                        )                                                                                                        n                i                            ⁡                              (                                  Ar                  +                                )                                      +                                          n                i                            ⁡                              (                                  CF                  4                  +                                )                                      +                                          n                i                            ⁡                              (                                                      C                    2                                    ⁢                                      F                    4                    +                                                  )                                      +            …                                              (        16        )            
In Expression (16), mi1 is the mass of ions produced by ionizing the first gas, such as Ar+ ions produced by ionizing Ar gas and mi2 is the reduced mass of ions dominant in the plasma produced by ionizing the second gas. If a single kind of ions are dominant, the reduced mass may be equal to the mass of the ions. If the plasma contains a plurality of kinds of dominant ions, the reduced mass is used. The value of ne1 is considered to be equal to that of ni+, the same can be calculated by using Expression (4).
Although it is possible to consider that Te1/Te2=1, Te1 and Te2 may be determined individually. In FIG. 1 showing the relation between current and voltage VP, the inclination of a section indicated by dotted line of a curve between a point where current is zero and a point in a positive region where current is saturated corresponds to 8kxc2x7Te1/xcfx80me. Therefore Te1 and Te2can be determined on the basis of the section of the curve.
To achieve the foregoing object, the present invention provides, on the basis of the foregoing principle, a method of measuring the negative ion density of a plasma, and a plasma processing method using the method of measuring the negative ion density of a plasma.
The method of measuring the negative ion density of a plasma according to the present invention comprises the steps of:
(a) supplying a first gas, which is an inert gas, into a vacuum chamber and ionizing the first gas to produce a first plasma;
(b) bringing the first plasma into contact with a probe having a base end connected through a variable-voltage power supply to a ground;
(c) measuring a saturation current Ies1 at which current flowing through the probe is saturated when the potential of the probe is changed by the variable-voltage power supply in a potential region where the potential of the probe is higher than a ground potential, and a saturation current Iis1 at which current flowing through the probe is saturated when the potential of the probe is changed by the variable-voltage power supply in a potential region where the potential of the probe is lower than the ground potential;
(d) supplying a second gas containing a gas for producing negative ions into the vacuum chamber and ionizing the second gas to produce a second plasma;
(e) bringing the second plasma into contact with the probe having the base end connected through the variable-voltage power supply to the ground;
(f) measuring a saturation current Ies2 at which current flowing through the probe is saturated when the potential of the probe is changed by the variable-voltage power supply in a potential region where the potential of the probe is higher than the ground potential, and a saturation current Iis2 at which current flowing through the probe is saturated when the potential of the probe is changed by the variable-voltage power supply in a potential region where the potential of the probe is lower than the ground potential; and
(g) determining the negative ion density ni1xe2x88x92 of the second plasma produced by ionizing the second gas by using Iis1/Iis2, Ies1/Ies2, mi1, mi2 and ne1, where mi1 is the mass of positive ions of the first gas, mi2 is the reduced mass of dominant positive ions among positive ions of the second gas and ne1 is the electron density of the first plasma.
The present invention provides also a plasma processing method using the method of measuring negative ion density of a plasma.
The present invention is applicable not only to elucidation of a state of a plasma but also to the control of a plasma processing system for processing a workpiece, such as a wafer, for a film forming process or an etching process using a plasma produced by subjecting a process gas to, for example, ECR. Control parameters, such as the pressure of the vacuum chamber and the flow rate of the gas, for controlling the plasma are controlled on the basis of the obtained (estimated) negative ion density. The method of ionizing the gas to produce a plasma is not limited to that which uses ECR, the method may be that which uses the energy of a microwave for ionizing a gas.
The present invention provides a plasma processing system that ionizes a process gas supplied into a vacuum chamber to produce a plasma, and uses the plasma for processing a workpiece, comprising: a probe having a base end connected through a variable-voltage power supply to a ground and disposed so as to come into contact with the plasma produced in the vacuum chamber; a current measuring device for measuring current that flows through the probe; a negative ion density measuring means for changing voltage applied to the probe by the variable-voltage power supply, sampling data on voltage applied to the prove and current that flows through the probe when an inert gas is ionized and when a mixed gas containing a process gas and an inert gas is ionized, and determining the negative ion density of a component of the process gas on the basis of the data; and a control parameter control means for controlling control parameters to be controlled to control a plasma on the basis of the negative ion density measured by the negative ion density measuring means.
The present invention provides also a negative ion density measuring apparatus comprising a probe having a base end connected through a variable-voltage power supply to a ground and disposed so as to come into contact with a plasma; a current measuring device for measuring a current that flows through the probe; and a negative ion density measuring means for changing voltage applied to the probe by the variable-voltage power supply, sampling data on voltage applied to the prove and current that flows through the probe when an inert gas is ionized and when a mixed gas containing a process gas and an inert gas is ionized, and determining the negative ion density of a component of the process gas on the basis of the data. The negative ion density of a plasma produced by ECR can be simply measured by the method of measuring negative ion density according to the present invention. Process conditions for a process for processing a workpiece by using a plasma can be minutely controlled by the plasma processing method and the plasma processing system according to the present invention for processing a workpiece in a high intrasurface uniformity.